The US 20070243634 A1 discloses a droplet microactuator with which single droplets of an emulsion can be manipulated. In a particular embodiment, magnetic beads that are contained in a droplet are immobilized at a surface of the device by a magnetic field. Another droplet can then be moved to their location, thus allowing to transfer the magnetic beads from one droplet to another. The mentioned apparatus is however rather complicated and hence not suited for many applications.